The present invention relates to a method of processing image information of an object to be reproduced. More particularly, the present invention relates to an image information processing method for processing color image signals from an object to be reproduced and/or for processing color image signals to be recorded in a digital color copying machine or a digital color printing machine.
In order to form color image signals with respect to an object such as a landscape, a document, light refelcted by the object is separated into a plurality of light of different colors usually into three (R, G and B) primary colors. Then, the separated light of different colors are received by photodetecting elements such as charge coupled devices (CCDs). The CCDs photoelectrically convert the light to color image signals proportional to the quantity of the reflected light of respective color. Since the magnitude of each of the color image signals is not proportional to a quantity of color perceived by a human, it is necessary to increase the S/N ratio of the color image signal. When such a signal is digitally processed, the number of bits of the signal increases because of redundancy. As a result, the processing device becomes complicated disadvantageously.
In order to decrease the redundancy in the signals, signals received by the CCDs are processed by a logarithmic amplifiers to generate color image signals proportional to the reflection density. However, as will be described in more detail later, even such signals being proportional to reflection density have much redundancy.
For expressing a color, CIE1976L*u*v* and CIE1976L*a*b* uniform color spaces are known. By using such colorimetric systems, it is possible to reduce the redundancy of the color signal. However, in order to convert the color signals received by the CCDs into color signals specifying color in thee systems, it is necessary to perform extremely complicated calculations among the color signals.
For the relationship between image signals to be recorded (recording image signals) and recorded density (reflectivity), so-called Yule-Nielsen equation is known. This Yule-Nielsen equation represents the relationship between a dot area rate a and the printed density D of a print by the following: EQU D=-nlog [1-a 1-10.sup.-Ds/n)]
where Ds is the solid inking density, and n is a constant which is set generally to 1 to 2 depending on the number of lines on the screen and a type of printing paper. In the field of plate making process, recording image signal is often expressed by such a dot area rate.
For recording by using a digital printer, a method is known which controls the gradation of record such that the relationship between recording image signals and recorded density becomes linear. This method is carried out by controlling the number of printed dots of a constant size or by changing the dot size.
There are no problems with the above-mentioned printing or recording methods if there are many usable recordable gradations. However, have the problem that if a recordable color space must be covered as uniformly as can with a limited number of gradations, unevenness would increase in the color space. If the number of gradations used is increased, a quantity of signal (the number of bits) to be processed would increase thereby render the processor complicated.